Inlet air filtration systems are generally employed for use with gas turbines and operate by removing salt, dust, corrosives and water (hereinafter referred to as “corrosives”) from inlet air in order to prevent their entry into the gas turbine. Corrosives can enter the gas turbines in various forms, such as solids (i.e., dry salt) or aqueous solutions (i.e., wet salt), and corrode the gas turbine elements. This corrosion could lead to operational failures and financial losses. As such, it is typically necessary to provide for corrosion mitigation to the gas turbine engine by way of an inlet filtration system that reduces the amount of corrosives entering the gas turbine.
The corrosives can exist in several states which can enter the gas turbine. One state includes Solid Particulate Corrosive elements. These include salt and oxide particles which can be removed by high efficiency filters. A second state includes Liquid, or, rather, Aqueous Corrosives elements. These include aqueous chlorides or acids, the removal of which cannot generally be efficiently accomplished by particulate filters.
In both cases, the corrosives can be moved along airflow by, typically, two main transfer mechanisms. These mechanisms include situations in which solid salts are deposited on particulate filters that can deliquesce when the humidity of ambient air rises beyond about 60% relative humidity (RH). The mechanisms may also include situations in which the filter elements themselves get wet and salts, which are in dissolved states via sea aerosol, rain, fog, mist and snow and other sources of water, enter the inlet air stream. Once salt solutions pass the final filters, there is a potential also for the liquid to dry and for salt to precipitate out of solution. This salt precipitate or crystallized salt can now enter the gas turbine, in addition to the liquid itself.
Current filtration systems available on the market specifically for salt and water removal are generally classified into 3-stage systems and barrier-type systems. The 3-stage systems include a first vane/moisture separator, as a first stage, coalescing filters, as a second stage, and a second vane separator as a third stage. The coalescing filter captures small salt aerosol droplets and causes them to coalesce into larger droplets and which can then be drained off as salt water. The coalescing filter also removes dust and dry salt particles from the inlet air which may be less than 1 micron in diameter and hydroscopic. The third stage removes any remaining droplets from the airstream, such as droplets that form from dry salt particles filtered by the coalescing filter, which take on water from humid inlet air and which are re-released into the airstream.
In a relatively dry environment in which the 3-stage system is used, a vane separator can be used as final stage, and dry salt particles may accumulate on the rear of the coalescing filter, due to successive periods of deliquescence. These dry salt particles can then be re-released into the airstream as fine particles which will not be removed by the second vane separator which can lead to salt accumulation on the gas turbine elements.
In the barrier-type systems, which are similar to two-stage static filter systems except that the final stage of the typical barrier-type system is a watertight high efficiency filter, the filter element is watertight and allows air, but not water, to pass through. They also do not include a second vane separator. In practice, barrier-type systems rely on a 100% seal against water being allowed to proceed downstream of the filter. This 100% seal is achievable on new and clean filtration systems but requires maintenance for proper operation. Therefore, while the barrier-type system media can be effective at stopping the migration of salt toward the gas turbine elements, the primary failure mode is seen as being the sealing mechanism if the maintenance or installation is performed improperly.